1. Field of the Invention
The present invention relates to a semiconductor device with a structure for detecting the magnitude of electric current that flows through the semiconductor device (electric current detecting function). Specifically, the present invention relates to a semiconductor device that is provided with the electric current detecting structure as well as a countermeasure against ESD (Electro Static Discharge).
2. Description of the Related Art
Generally, a semiconductor device is provided with a semiconductor substrate having a plurality of switching elements formed within the substrate. The switching element in this specification means a structure composed of semiconductor regions having different characters that switches on/off the electric current that flows between a pair of electrodes.
A semiconductor device having electric current detecting function has been developed. Typically, in order to detect the magnitude of the electric current that flows through the semiconductor device, the switching elements are partitioned into two groups. Specifically, by partitioning the electrode on the lower electric potential side into two, the switching elements become partitioned into two groups. A portion of the switching elements is electrically connected in common to one of the two electrodes, and the other portion of the switching elements is electrically connected in common to the other electrode. Accordingly, the switching elements are partitioned into two groups: a group connected to one of the two electrodes and a group connected to the other electrode. The electrode of the former is connected to a reference potential, and the electrode of the latter is connected to the reference potential via a current detector. In the present specification, the former electrode is referred to as “main electrode,” and the latter electrode is referred to as “sensor electrode.” Further, the switching elements that turn on/off the electric current that flows through the main electrode are referred to as “main switching elements,” and the switching elements that turn on/off the electric current that flows through the sensor electrode are referred to as “sensor switching elements.” The main switching elements and sensor switching elements are electrically connected in common to the electrode on the higher electric potential side though an electric device such as a motor. A common gate voltage is applied to gate electrodes of the main switching elements and sensor switching elements.
According to this semiconductor device, when a gate-on voltage is applied to the gate electrodes, the main switching elements and sensor switching elements both turn on, and the electric current flows through both the main switching elements and sensor switching elements. The magnitude of the electric current that flows through each switching element is same among the main switching elements and sensor switching elements. Therefore, a proportional relationship between the number of the main switching elements and the number of the sensor switching elements become equal to a proportional relationship between the magnitude of the electric current that flows through the main electrode and the magnitude of the electric current that flows through the sensor electrode. Therefore, by detecting the magnitude of the electric current that flows through the sensor electrode, the magnitude of electric current that flows through the main electrode can be derived.
By detecting the magnitude of electric current that flows through the semiconductor device, it becomes possible to detect a phenomena that an abnormally large current flows the semiconductor device. This phenomena is caused when the electric device connected to the semiconductor device is short-circuited. Accordingly, by providing the sensor switching elements, the sensor electrode, and a means for forcing the semiconductor device to turn off when the detected electric current exceeds a predetermined value, excessive current higher the predetermined value can be prevented from flowing through the semiconductor device, even when the electric device connected to the semiconductor device is short-circuited.
Generally, the number of the sensor switching elements is much smaller than the number of the main switching elements. Therefore, the gate input capacity (gate input capacity correlates to the area of a gate insulating layer) between the sensor electrode and gate electrodes of the sensor switching elements is smaller than the gate input capacity between the main electrode and gate electrodes of the main switching elements. Since the gate input capacity of the main switching elements is large, the ESD tolerance between the main electrode and gate electrodes of the main switching elements is high. On the other hand, since the gate input capacity of the sensor switching elements is small, the ESD tolerance between the sensor electrode and gate electrodes of the sensor switching elements is low. Therefore, when ESD is applied between the sensor electrode and gate electrodes of the sensor switching elements, the gate insulating layers of the sensor switching elements are easily destroyed.
In order to improve the ESD tolerance between the sensor electrode and gate electrodes of sensor switching elements, a technique is known that provides a zener diode between the sensor electrode and gate electrode. When ESD that reaches the breakdown voltage of the zener diode is applied between the sensor electrode and gate electrode, the zener diode experiences a breakdown. As a result, a voltage that is higher than the breakdown voltage of the zener diode is prevented from being applied to the gate insulating layers of the sensor switching elements. Therefore, the gate insulating layers of the sensor switching elements can be prevented from being destroyed.
The techniques associated with this type of technique are described in Japanese Laid-Open Patent Application Publication No. 1994-85174, Japanese Laid-Open Patent Application Publication No. 2001-358568, Japanese Laid-Open Patent Application Publication No. 2001-16082, and Japanese Laid-Open Patent Application Publication No. 2002-517116.